The invention is directed to dispersion compensation fibers for use in telecommunication systems, and more particularly, to optical fibers for compensating the dispersion and the dispersion slope of non-zero dispersion shifted fibers.
The increased demand for higher bit transmission rates has resulted in a large demand for optical transmission systems that can control dispersion effects. A linear analysis of common optical transmission systems indicates that while transmission systems can tolerate about 1,000 ps/nm residual dispersion at 10 Gbit/second, these systems tolerate only about 62 ps/nm residual dispersion at a higher transmission rate of 40 Gbit/second. Therefore, it is apparent that it is important to accurately control the dispersion for high bit-rate transmission systems, and that this control becomes increasingly important as the transmission rate increases. Further, the need to accurately control dispersion means that dispersion slope of a transmission fiber must also be compensated as transmission rates approach 40 Gbit/second.
Various solutions have been proposed to achieve the low dispersion and dispersion slope values required for compensating non-zero dispersion shifted fibers, including: photonic crystal fibers, higher order mode dispersion compensation, dispersion compensating gratings and dual fiber dispersion compensating techniques. Each of these solutions has significant drawbacks associated therewith.
Photonic crystal fibers are designed to have a large negative dispersion and a negative dispersion slope that are close to those required for compensating non-zero dispersion shifted fibers. However, photonic crystal fibers have significant drawbacks including a relatively small effective area of about 10 xcexcm2 or less that leads to unacceptably high splice losses and hence require the use of a transition or bridge fiber to reduce splice losses. In addition, due to the very nature of photonic crystal fibers, i.e. glass/air interfaces in the core of the fiber, the related attenuation is unacceptable in the transmission window of interest due. Further, photonic crystal fibers are significantly difficult to manufacture on a large scale and are, therefore, expensive.
Higher order dispersion mode compensation relies on the dispersion properties of higher order modes in the fiber. It has been demonstrated that higher order modes, e.g. LP02 and LP11, have higher negative dispersions and dispersion slopes than fundamental modes. Higher order mode dispersion compensation typically relies on the conversion of a transmitted fundamental mode to one of the higher order modes via a mode converter. Subsequently, this higher order mode is propagated in a fiber. After a finite distance, the higher order mode is converted back to the fundamental mode via a second mode converting device. Problems associated with higher order mode dispersion compensation solutions include inefficient mode converters and the difficulty of producing intermediate fibers that allow higher order mode transmission while resisting coupling to the fundamental mode.
Dispersion compensating gratings are utilized to achieve a required differential group delay via chirped gratings. Techniques utilizing dispersion compensating gratings have been shown to be useful only for narrow bands, as these techniques typically suffer from dispersion and dispersion slope ripple when the required grating length becomes large.
Dual fiber dispersion compensating solutions for non-zero dispersion shifted fibers are similar to the dispersion compensating gratings techniques described above in that the dispersion compensation and the slope compensation are separately treated. Typically, dual fiber dispersion compensating techniques include the use of a dispersion compensation fiber followed by a dispersion slope compensation fiber. Such solutions require the use of a dispersion slope compensation fiber that compensates for a relatively small dispersion slope. Extensive profile modeling of optical fibers has resulted in well-established correlation between dispersion slope, effective area and bend sensitivity. By increasing the role played by waveguide dispersion in a given fiber, it is possible to decrease the slope and even create a negative slope in some cases. However, as the effective area is decreased, the bend sensitivity of the fiber is increased. Effective area of the fiber can be increased at the expense of further degradation of the bend sensitivity. Decreasing the dispersion slope, or making the dispersion slope negative, results in working very close to the cut-off wavelength of the fundamental mode, which in turn makes the fiber more bend sensitive and results in greater signal loss at long wavelengths, i.e., wavelengths greater than 1560 nm. As a result of these relationships, it is extremely difficult to manufacture a viable compensation fiber that compensates both dispersion and dispersion slope.
Heretofore, the most viable broad band commercial technology available to reduce or eliminate dispersion has been dispersion compensation fiber modules. As dense wavelength division multiplexing deployments increase to 16, 32, 40 and more channels, broadband dispersion compensating products are desired. Telecommunication systems presently include single-mode optical fibers which are designed to enable transmission of signals at wavelengths around 1550 nm in order to utilize the effective and reliable erbium-doped fiber amplifiers currently available.
With continuing interest in higher bit-rate information transfer, i.e. greater than 40 Gbit/second, ultra-long reach systems, i.e., systems greater than 100 km in length, and optical networking, it has become imperative to use dispersion compensation fibers in networks that carry data on non-zero dispersion shifted fibers. One such fiber is LEAF(copyright) optical fiber, manufactured by and available from Coming, Inc. of Coming, N.Y. LEAF(copyright) fiber is a positive non-zero dispersion shifted fiber that has become the optical fiber of choice for many new systems due to its inherently low dispersion and economic advantage over conventional single mode fibers. The dispersion curve for LEAF(copyright) fiber is very linear as a function of wavelength over the C and L wavelength bands, with a zero-dispersion wavelength of around 1501 nm. The linearity of the dispersion relation makes broadband dispersion compensation difficult, since the dispersion curves of prior art dispersion compensation fibers are only approximately linear. Specifically, the profiles of prior art dispersion compensation fibers previously used to compensate LEAF(copyright) fiber each suffer from rather severe dispersion curvature as a function of wavelength that limits their application bandwidth.
The combination of the early versions of dispersion compensation fibers with non-zero dispersion shifted fibers effectively compensated dispersion at only one wavelength. However, higher bit-rates, longer reaches and wider bandwidths require dispersion slope to be more precisely compensated over broad ranges of wavelength. Consequently, it is desirable for the dispersion compensation fiber to have dispersion characteristics such that its dispersion and dispersion slope are matched to that of the transmission fiber it is required to compensate over broad wavelength ranges. The dispersion and dispersion slope matching characteristic described above is also indicative of multiple dispersion compensation fiber systems.
Thus, it should be recognized that there is a need to compensate dispersion across the entire wavelength band of operation and not just at wavelengths near the center of the band as is common with many of the solutions currently being employed. Therefore, it would be desirable to develop an alternative dispersion compensating fibers and transmission systems having the ability to compensate for dispersion and dispersion slope of non-zero dispersion shifted fibers and other positive dispersion optical fibers over a wide wavelength band.
The present invention relates to a dispersion compensating fiber that compensates for dispersion and dispersion slope of a non-zero dispersion shifted fiber (NZDSF). The methods and apparatus disclosed herein enable accurate and substantially complete compensating for dispersion and dispersion slope in a NZDSF across a wide wavelength band.
One embodiment of the present invention relates to a dispersion and dispersion slope compensating optical fiber that includes a segmented core and a cladding layer on the periphery of the core, wherein the fiber""s refractive index profile is selected to provide a dispersion having a maximum deviation of less than 7 ps/nm-km within a wavelength band of from about 1550 nm to about 1610 nm; more preferably, a maximum deviation of less than 5 ps/nm-km within the wavelength band of from about 1550 nm to about 1610 nm. The DC fiber preferably exhibits a fundamental mode bend loss of less than or equal to 0.01 dB/km within the wavelength band of from about 1550 nm to about 1610 nm. Preferably also, the DC fiber according to the invention exhibits an effective area greater than or equal to 17 xcexcm2 at 1580 nm. The DC fiber in accordance with the invention preferably exhibits a total dispersion more negative than xe2x88x9250 ps/nm-km at 1550 nm; preferably more negative that xe2x88x9275; and more preferably more negative that xe2x88x92120 ps/nm-km. The DC fiber in accordance with the invention preferably exhibits a total dispersion slope more negative that xe2x88x922 ps/nm2-km at 1580 nm.
In a first preferred embodiment, a segmented core of the dispersion compensating fiber includes a central core segment having a relative refractive index, a depressed moat segment on the periphery of the central core segment and having a relative refractive index that is less than the relative refractive index of the core segment, and an outer radius. The segmented core also preferably includes an intermediate segment on the periphery of the depressed moat segment and having a relative refractive index that is less than the relative refractive index of the central core segment and greater than the relative refractive index of the depressed moat segment, and an outer radius. The segmented core preferably further includes an annular ring segment on the outward of the moat and preferably located on the periphery of the intermediate segment and having a relative refractive index that is greater than the relative refractive index of the intermediate segment, and a width.
In a second preferred embodiment, the segment core of the dispersion compensating fiber includes a central core segment having a relative refractive index, and a depressed moat segment on the periphery of the central core segment and having a relative refractive index that is less than the relative refractive index of the central core segment, and an outer radius. The segmented core preferably also includes a first intermediate segment on the periphery of the depressed moat segment and having a relative refractive index that is less than the relative refractive index of the central core segment and greater than the relative refractive index of the depressed moat segment, and an outer radius, and an annular ring segment on the periphery of the first intermediate segment and having a relative refractive index that is greater than the relative refractive index of the first intermediate segment and less than the relative refractive index of the central core segment, and a width. The segmented core further preferably includes a second intermediate segment on the periphery of the ring segment and having a relative refractive index that is less than the relative refractive index of the central core segment and greater than the relative refractive index of the moat segment, and an outer radius, and a gutter segment on the periphery of the second intermediate segment and having a relative refractive index that is less than the relative refractive index of the second intermediate segment and greater than the relative refractive index of the depressed moat segment, and an outer radius. The segmented core still further includes an outer clad on the periphery of the gutter segment and having a relative refractive index that is greater than the relative refractive index of the moat segment and less than the relative refractive index of the second intermediate segment.
According to a further embodiment, a dispersion compensating optical fiber is provided having a refractive index profile with a central core segment having a positive relative refractive index; a depressed moat segment on a periphery of the central core segment and having a relative refractive index that is more negative that xe2x88x921.2%; and an annular ring segment outward from the depressed moat segment and having a relative refractive index that is greater than 1.2%.
The present invention also includes optical communication systems employing dispersion compensation fibers and modules in accordance with the embodiments described above.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows, together with the claims and appended drawings.
It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for understanding the nature and character of the invention as it is defined in the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention, which, together with their description serve to explain the principles and operation of the invention.